Switching regulators usually operate in either a continuous conduction mode (CCM) for higher power applications, or in a discontinuous conduction mode (DCM) for lower power applications. For example, when a switching regulator operates in CCM, current in its inductive element is always above zero, whereas in a DCM switching regulator, current in the inductive element falls to zero during a time period within each switching cycle.
To take full advantage of energy transfer efficiency, most flyback switching regulators operate in DCM where an inherent diode reverse recovery problem is absent and switches operate at zero current turn-on. Due to reduced device stresses, a DCM switching regulator is able to operate at a higher frequency, which results in a smaller transformer size and higher power density. However, DCM operation is associated with high resistive loss and high ripple voltage. To minimize these disadvantages while maintaining advantages of DCM operation, a switching regulator may be controlled to operate at the boundary between CCM and DCM, i.e. in a boundary conduction mode (BCM).
The efficiency of a BCM regulator is higher than that of a DCM regulator. However, the difference in efficiency between these modes becomes much smaller at high line voltage conditions. In addition, BCM operation has some disadvantages. A BCM controller adjusts switching frequency according to load and line conditions. Load step change and line voltage variation affect the switching frequency directly. The resulting variable switching frequency may shift the noise spectrum out of an acceptable range of electromagnetic interference (EMI) and cause uncertainty for EMI filtering.
Variable switching frequency also affects output voltage ripple. When the switching frequency is high, output voltage ripple is low. Conversely, when switching frequency is low, output voltage ripple is high.
Another disadvantage is “jittering” at light load conditions, when the switching frequency can reach a very high level.
DCM operation enables a switching regulator to alleviate the EMI and “jittering” problems and conveniently provide ripple filtering. Therefore, it would be desirable to provide a conduction mode control system that will allow a switching regulator to combine benefits of BCM and DCM by having a capability to operate in either mode, selectively. Also, it would be desirable to control a switching converter so as to support its operation in a hybrid DCM/BCM mode, in which the converter is able to switch between BCM and DCM, depending on specific conditions.